This invention relates to DNA coding for a signal amino acid sequence which acts to secrete products out of cells and to DNA containing such a sequence.
The signal amino acid sequence herein means an amino acid sequence acting so that products which have been formed in cells can be secreted out of the cells. Generally, products are produced in cells and accumulated therein. On the other hand, those products which have a signal amino acid sequence are said to be secreted out of cells along with their formation therein. Accordingly, products in cells can successfully be secreted out of the cells if such a signal amino acid sequence is utilized.
Secretion of products out of cells could have the following merits: Firstly, if products in cells can be moved out of the cells, it will become possible to separate impurities from the products easily and to reduce the labor required for their purification and isolation procedures. In addition, the products in cells can be isolated in a pure state without being accompanied by poisonous substances contained in the cell membrane, so that they will have uses in a wide range, with no restriction in their application. Secondly, even when products in cells are such that their production in cells would be inhibited by their own excessive formation, they will be freed from the feedback inhibition as they are transferred out of the biosynthetic system, so that their excessive production will become possible. Thirdly, those products in cells which are harmful to the growth of cells can be moved out of the cells, so that their production can be carried on while the cells are maintained wholesome.
Several kinds of such signal amino acid sequences and DNA base sequences thereof have been known. They are, for example, a signal amino acid sequence for penicillinase for Bacillus licheniformis (Nucleic Acid Research Vol. 9, No. 11, 2577 (1981)) and a signal amino acid sequence for .alpha.-amylase for Bacillus amyloliquefaciense (Gene, 15, 43 (1981)).
The present inventors have carried out the cloning of .alpha.-amylase gene of Bacillus subtilis known for its very high .alpha.-amylase productivity, analyzed such cloned gene, and discovered a new signal amino acid sequence different from the known signal amino acid sequence and DNA base sequence thereof for amylase. As a result, they have invented a new signal amino acid sequence and DNA base sequence different from those heretofore known.
When production in cells is carried out using the DNA of this invention in a host vector system where the host is Bacillus subtilis which has a high .alpha.-amylase productivity, the system in this invention will be superior in stability and secretory productivity to the system used a different organism or a different signal amino acid sequence.